This application claims priority from Canadian Patent Application No. 2,361,969 filed Nov. 14, 2001, and incorporated herein by reference.
The present invention relates to a method and apparatus using digital imaging and processing for detecting overlapped flat objects in a sequence of flat objects. More particularly, the invention is applicable to the detection of double or multiple fed mail pieces in a mail sorting apparatus.
Mechanisms for minimizing multiple feeds when processing a stack of flat objects are well known. For example, sheet feeders, bank note readers and mail piece sorting systems all employ feed mechanisms for picking off work pieces sequentially and singly from an input stack for transport along a feed path at relatively high speed.
In a mail sorting system, the mail pieces are essentially flat rectangular objects having a pair of large flat surfaces and four edges, and the mail pieces are arranged with their planar surfaces along a common axis to form a stack.
A feeder mechanism picks off individual mail pieces from an input stack to an OCR (Optical Character Reader) which reads a forwarding address printed on the mail piece and directs the mail piece to one of several output stacks corresponding to the destination address. The feed rate of such sorting apparatus is typically several thousand mail pieces per hour, so occasionally more than one mail piece is picked off by the feeder resulting in a multiple feed, also referred to in the art as a double feed. Multiple feeds pose a problem in that two or more mail pieces may end up in the wrong destination stack with the result that the misfed mail pieces are not delivered on time. Furthermore, multiple fed mail pieces may cause jamming within the stacker apparatus. Both of these problems are costly. Accordingly, the benefits of detecting multiple fed mail pieces are evident, and particularly if the multiple feeds are detected as early as possible in the feed path.
A xe2x80x9cdouble feedxe2x80x9d is characterized by two or more mail pieces being stuck together generally along their flat sides with either one or more edges completely or partially overlapped. While current double feed detection systems will detect a partial or complete overlap, few are capable of also distinguishing a false double feed, which occurs when a relatively thick mail piece with a crinkled or creased edge is picked off or when the mail piece has a dark color, is multicolored or has a fold over. Of course, the detection of false double feeds should be avoided.
There are a number of disadvantages with current techniques for detecting double feeds. For example, U.S. Pat. No. 4,733,226 describes a system for detecting overlapped mail pieces where one mail piece hides another in the feed path. A scanner is arranged along the feed path to detect the height of the mail piece as it moves past the scanner. Any changes in the height of mail pieces signals an overlap condition. As the system is limited to detecting variations in height, it cannot be used to detect an overlap where the mail pieces are the same height or where the mail pieces are fully overlapped. In U.S. Pat. No. 4,160,546, there is described a system which uses changes in document translucency to trigger an overlap indication. While this system may be effective for detecting documents that are translucent and have similar characteristics, it is not as effective for mail pieces which are typically opaque.
Also, imaging techniques for counting stacks of flat objects are known, however these techniques have limitations when used for double feed detection. For example, U.S. Pat. No. 5,534,690 describes a system for counting the number of bank notes in a stack by imaging the entire side of a stack while the stack is kept stationary. The system determines the number of items in the stack by taking two images of the side of the stack at different illuminations. The number of lines in the two images is compared. The average number of lines between the two images indicates the number of items in the stack. A limitation of this system is that the stacked items must be stationary so that a meaningful comparison can be made between the two images. Accordingly, this technique cannot be used for determining double feeds in a moving stream of objects such as in a mail sorting apparatus. Other patents that describe counting techniques are, for example, disclosed in U.S. Pat. No. 5,221,837.
A need therefore exists for the effective detection of double feeds, including both partially overlapped and fully overlapped mail pieces, in a mail sorting and handling apparatus, while the mail pieces are in motion. Furthermore, there is a need for a double feed detection (DFD) system that can detect double feeds where the objects have different heights, different colors, and different widths and with crinkled edges with minimal impact on feed mechanisms or existing sorters.
The invention provides a double feed detection system and method for detecting two or more mail pieces (e.g. envelopes), either partially or fully overlapped, passing simultaneously through a mail sorting and handling apparatus.
The DFD system includes a vision system with a digital camera for capturing and analyzing images of the bottom edges of mail pieces as they pass through the mail sorting apparatus, multiple photosensors for detecting and tracking the mail pieces through the mail sorting apparatus, and a controller for system control, system fault monitoring, and outputting double feed rejection signals to the mail sorting apparatus to enable the re-routing of detected double feeds.
In particular, according to one aspect of the invention, a system is provided for detecting overlapped flat objects in a sequence of flat objects, where the flat objects have at least one of their edges exposed for viewing as they pass along a feed path. The system includes: a sensor for generating a signal in response to detecting a flat object in the feed path; a camera responsive to the signal for capturing a digital image of the exposed edges of the detected flat object in the feed path; and a vision system coupled to the camera for receiving the digital image. The vision system analyzes at least a portion of the image to determine a pixel density variation along a direction perpendicular to the edges and uses the pixel density variation to output an indication of the number of edges in the image.
The DFD method is implemented in part by software run by the vision system. According to this method, an image of the bottom edges of a mail piece is captured and an inspection is performed on at least a portion of this image to determine if the mail piece is of a predetermined thickness. If the mail piece is of the predetermined thickness, low sensitivity settings of the expected average edge width are used by the software to count the number of edges. If the mail piece is less than the predetermined thickness, high sensitivity settings of the expected average edge width are used to count the number of edges. If the mail piece is of the predetermined thickness and the measured number of edges is less than two, there is no double feed and an output from the vision system to the controller indicates an xe2x80x9cOKxe2x80x9d condition for the mail piece. On the other hand, if the mail piece is of the predetermined thickness and the measured number of edges is not less than two, there is a double feed and the output to the controller indicates a xe2x80x9cDouble Feedxe2x80x9d condition for the mail piece. If the mail piece is less than the predetermined thickness and the measured number of edges is less than two, there is no double feed condition and the output to the controller indicates an xe2x80x9cOKxe2x80x9d condition for the mail piece. If the mail piece has less than the predetermined thickness and the measured number of edges is greater than two, there is a double feed and the output to the controller indicates a xe2x80x9cDouble Feedxe2x80x9d condition for the mail piece. Finally, if the mail piece has less than the predetermined thickness and the measured number of edges is equal to two and the measured edge pitch is smaller than a predetermined threshold, there is no double feed and the output to the controller indicates an xe2x80x9cOKxe2x80x9d condition for the mail piece. On the other hand, if the mail piece has less than the predetermined thickness, the measured number of edges is equal to two and the measured edge pitch is greater than a predetermined threshold, there is a double feed and the output to the controller indicates a xe2x80x9cDouble Feedxe2x80x9d condition for the mail piece.
In particular, according to another aspect of the invention, a method is provided for detecting overlapped flat objects in a sequence of flat objects where the flat objects have at least one of their edges exposed for viewing as they pass along a feed path. The method includes the steps of: selecting a flat object in the feed path; capturing a digital image of the exposed edges of the selected flat object; processing at least a portion of the captured image encompassing the edges to determine a pixel density variation in a direction across the edges; analyzing the pixel density variation to identify maxima and minima in the variation, where a start of an edge is identified by a maximum and an end of an edge is identified by a minimum; and, counting the maxima and minima to output an indication of the number of edges in the image. The method may further include determining an edge width of the flat object. The method of determining an edge width of the flat object may include: computing an average pixel density for the processed portion; assuming a first edge width if the average density is below a predetermined level; and, assuming a second edge width if the average density is above the predetermined level. The method of counting may further include: counting maximum and minimum pairs that are spaced by less than the first edge width if the average density is below the predetermined level to output the indication of the number of edges; and, counting maximum and minimum pairs that are spaced by more than the second edge width if the average density is above the predetermined level to output the indication of the number of edges.
The DFD method is also implemented in part by software that is run on the controller. The controller receives outputs from the vision system indicating that each passing mail piece is either xe2x80x9cOKxe2x80x9d or is a xe2x80x9cDouble Feedxe2x80x9d. The controller analyzes these outputs from the vision system, information from multiple photosensors that track the progress of mail pieces through the mail sorting, and monitored fault information to determine when or if a double feed rejection signal should be sent to the mail sorting apparatus to enable the re-routing of detected double feeds.